Muscle Tissue and Physiology

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is classified into three types based on structure, location, and function.

• Skeletal Muscle: Attached to bones, voluntary control, striated appearance.

• Cardiac Muscle: Found in the heart, involuntary control, striated, intercalated discs present.

• Smooth Muscle: Located in walls of hollow organs (e.g., intestines, blood vessels), involuntary, non-striated.

Properties of Muscle Cells

• Excitability: Ability to respond to stimuli.

• Conductivity: Ability to propagate electrical signals.

• Contractility: Ability to shorten forcefully.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to return to original length after stretching.

Skeletal Muscle Fiber Structure

• Sarcoplasmic Reticulum (SR): Stores and releases calcium ions for contraction.

• T Tubules: Invaginations of the sarcolemma that transmit action potentials into the cell.

• Terminal Cisternae: Enlarged areas of the SR adjacent to T tubules; part of the triad.

• Triad: Structure formed by a T tubule and two terminal cisternae.

• Sarcomere: Functional contractile unit of muscle fiber, defined by Z discs.

• Mitochondria: Provide ATP for muscle contraction.

• Myofibrils: Bundles of myofilaments (actin and myosin) within muscle fibers.

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm of muscle fiber, contains glycogen and myoglobin.

Functions of Skeletal Muscle Fiber Structures

• Sarcoplasmic Reticulum: Regulates intracellular calcium concentration.

• T-Tubules: Conduct action potentials to the interior of the muscle fiber.

• Sarcomere: Site of muscle contraction via sliding filament mechanism.

• Sarcolemma: Initiates and conducts action potentials.

• Sarcoplasm: Contains organelles and energy reserves.

• Glycogen: Storage form of glucose for energy.

• Mitochondria: ATP synthesis via aerobic respiration.

• Myoglobin: Oxygen-binding protein for aerobic metabolism.

Myofilament Organization and Sarcomere Structure

• Thick Filaments: Composed of myosin proteins.

• Thin Filaments: Composed of actin, troponin, and tropomyosin.

• Sarcomere Bands and Lines:

  • I Band: Light band, contains only thin filaments.

  • A Band: Dark band, contains thick filaments and overlapping thin filaments.

  • H Zone: Central region of A band, only thick filaments.

  • M Line: Center of H zone, holds thick filaments together.

  • Z Disc: Boundary of sarcomere, anchors thin filaments.

  • Connectin (Titin): Elastic protein, stabilizes thick filaments and provides elasticity.

  • Dystrophin: Links thin filaments to sarcolemma, important for structural integrity.

Thin and Thick Filament Details

• Thin Filaments:

  • Actin: Has myosin binding sites.

  • Troponin: Binds calcium, regulates position of tropomyosin.

  • Tropomyosin: Covers myosin binding sites on actin at rest.

• Thick Filaments:

  • Myosin: Has heads that bind actin and hydrolyze ATP.

  • Myosin Head: Binds to actin and ATP, performs power stroke.

Sliding Filament Mechanism of Contraction

1. Calcium binds to troponin, causing tropomyosin to move and expose myosin binding sites on actin.

2. Myosin head hydrolyzes ATP and binds to actin (crossbridge formation).

3. Power stroke: Myosin head pivots, pulling actin filament.

4. Myosin head releases actin (detachment) and resets (return) with new ATP.

5. During contraction: I band and H zone shorten, A band remains the same length.

Connective Tissue Organization

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds fascicles (bundles of fibers).

• Epimysium: Surrounds the entire muscle.

Neuromuscular Junction (NMJ) and Muscle Excitation

• Motor Neuron: Transmits action potential to muscle fiber.

• Action Potential: Electrical signal that triggers neurotransmitter release.

• Synaptic Vesicles: Contain acetylcholine (ACh).

• Synaptic Cleft: Gap between neuron and muscle fiber.

• ACh: Binds to receptors on motor end plate, initiating muscle action potential.

• Acetylcholinesterase: Enzyme that degrades ACh, ending stimulation.

Muscle Action Potential and Contraction

• Binding of ACh to receptors causes depolarization of sarcolemma.

• Action potential travels along sarcolemma and down T-tubules.

• Triggers calcium release from sarcoplasmic reticulum.

• Calcium initiates contraction cycle.

Muscle Relaxation

• ACh receptors close; ACh is removed by acetylcholinesterase.

• Calcium channels on SR close; calcium is pumped back into SR.

• ATP binds to myosin head, causing detachment from actin.

• Troponin-tropomyosin complex reforms, covering myosin binding sites.

ATP Generation in Muscle Cells

• Creatine Phosphate Pathway: Immediate ATP supply.

• Anaerobic Respiration: Short-term ATP supply, produces lactic acid.

• Aerobic Respiration: Long-term ATP supply, requires oxygen.

Muscle Twitch and Contraction Types

• Muscle Twitch Phases:

  • Latent Period

  • Contraction Period

  • Relaxation Period

• Isometric Contraction: Muscle length does not change.

• Isotonic Contraction: Muscle changes length (concentric or eccentric).

Muscle Fiber Types and Motor Units

• Fast-Twitch vs. Slow-Twitch Fibers: Fast-twitch contract quickly, fatigue rapidly; slow-twitch contract slowly, resist fatigue.

• Oxidative vs. Glycolytic Fibers: Oxidative use aerobic metabolism; glycolytic use anaerobic metabolism.

• Motor Unit: A motor neuron and all muscle fibers it innervates.

• Motor Unit Recruitment: Increases force of contraction.

• Inverse Relationship: Smaller motor units allow finer control.

Muscle Tone, Hypertrophy, Fatigue, and EPOC

• Muscle Tone: Continuous, partial contraction of muscles.

• Hypertrophy: Increase in muscle size due to increased fiber size.

• Muscle Fatigue: Inability to maintain contraction; caused by ATP depletion, lactic acid buildup, etc.

• Excess Post-Exercise Oxygen Consumption (EPOC): Increased oxygen intake after exercise to restore metabolic conditions.

Smooth Muscle

• Location: Walls of hollow organs, blood vessels, respiratory tract.

• Single-Unit vs. Multiunit: Single-unit contracts as a group; multiunit contracts independently.

• Filament Arrangement: No sarcomeres; filaments arranged diagonally.

• Smooth Muscle Contraction Steps:

  1. Calcium enters cell.

  2. Calcium binds to calmodulin.

  3. Activates myosin light-chain kinase.

  4. Phosphorylates myosin heads.

  5. Crossbridge cycling and contraction.

Muscular System: Axial and Appendicular Muscles

Muscle Attachments and Actions

• Origin: Fixed attachment point of muscle.

• Insertion: Movable attachment point.

• Agonist: Prime mover of an action.

• Antagonist: Opposes the action of the agonist.

• Synergist: Assists the agonist.

• Fixator: Stabilizes the origin of the agonist.

Muscle Naming Criteria

• Location (e.g., brachialis)

• Shape (e.g., deltoid)

• Size (e.g., gluteus maximus)

• Direction of fibers (e.g., rectus abdominis)

• Number of origins (e.g., biceps brachii)

• Location of attachments (e.g., sternocleidomastoid)

• Action (e.g., flexor carpi radialis)

• Combination of above

Nervous System: Nervous Tissue

Functions and Divisions of the Nervous System

• Functions:

  1. Sensory input

  2. Integration

  3. Motor output

• Structural Divisions:

  • Central Nervous System (CNS): Brain and spinal cord

  • Peripheral Nervous System (PNS): Nerves and ganglia

• Functional Divisions:

  • Somatic sensory

  • Visceral sensory

  • Somatic motor

  • Visceral motor (autonomic)

Neuron Structure and Function

• Cell Body: Contains nucleus and organelles.

• Chromatophilic Substance (Nissl Bodies): Rough ER, synthesizes proteins.

• Dendrites: Receive signals.

• Axon: Transmits signals away from cell body.

• Axon Hillock: Initiates action potential.

• Collateral: Branch of axon.

• Node of Ranvier: Gap in myelin sheath, facilitates saltatory conduction.

• Synaptic Knobs: Release neurotransmitters.

• Synaptic Vesicles: Store neurotransmitters.

Neuron Classification

• Structural: Multipolar, bipolar, unipolar.

• Functional: Sensory (afferent), motor (efferent), interneurons.

Glial Cells

• CNS:

  • Astrocytes: Form blood-brain barrier.

  • Ependymal cells: Produce cerebrospinal fluid.

  • Microglia: Phagocytosis of debris.

  • Oligodendrocytes: Myelinate CNS axons.

• PNS:

  • Schwann cells: Myelinate PNS axons.

  • Satellite cells: Support neuron cell bodies in ganglia.

Myelin, Gray and White Matter

• Myelin: Lipid-rich sheath, insulates axons, increases conduction speed.

• Gray Matter: Neuron cell bodies, dendrites, unmyelinated axons.

• White Matter: Myelinated axons.

Neuron Regeneration (PNS)

• Occurs if cell body is intact and Schwann cells are present.

Ion Channels and Membrane Potentials

• Channel Types: Leak channels, gated channels (chemically, voltage, mechanically gated).

• Resting Membrane Potential: Maintained by Na+/K+ pump; more Na+ outside, more K+ inside.

• Graded Potential: Local change in membrane potential.

• Depolarization: Membrane potential becomes less negative (Na+ influx).

• Hyperpolarization: Membrane potential becomes more negative (K+ efflux or Cl- influx).

• EPSP: Excitatory postsynaptic potential, depolarizes membrane.

• IPSP: Inhibitory postsynaptic potential, hyperpolarizes membrane.

• Threshold: Minimum membrane potential to trigger action potential.

• Summation: Temporal (same synapse, rapid succession) or spatial (multiple synapses).

Action Potential

• All-or-None Law: Action potential occurs fully or not at all.

• Phases:

  • Resting potential

  • Threshold

  • Depolarization (Na+ channels open)

  • Repolarization (K+ channels open)

  • Hyperpolarization

• Refractory Periods:

  • Absolute: No new action potential possible.

  • Relative: Stronger stimulus needed for new action potential.

• Conduction:

  • Saltatory: Jumps between nodes of Ranvier (myelinated).

  • Continuous: Along unmyelinated axons.

• Conduction Velocity: Faster in larger, myelinated fibers.

Synapses and Neurotransmitters

• Electrical Synapse: Direct ion flow via gap junctions.

• Chemical Synapse: Neurotransmitter release into synaptic cleft.

• Neurotransmitter: Chemical messenger released from synaptic vesicles; calcium influx triggers release.

• Examples:

  • Norepinephrine: Excitatory or inhibitory, autonomic functions.

  • Acetylcholine: Excitatory at NMJ, inhibitory in heart.

  • Dopamine: Motor control, reward pathways.

  • GABA: Main inhibitory neurotransmitter in CNS.

  • Glutamate: Main excitatory neurotransmitter in CNS.

  • Serotonin: Mood regulation, sleep.

• Neurotransmitter Degradation: Enzymatic breakdown or reuptake terminates signal.

Neuronal Circuits

• Converging Circuit: Multiple inputs, one output.

• Diverging Circuit: One input, multiple outputs.

• Neuronal Pool: Group of interconnected neurons with specific function.

Key Equations

• Resting Membrane Potential: Additional info: This is the Nernst equation for potassium; actual resting potential is determined by all ions present.

• ATP Hydrolysis:

Sample Table: Muscle Types Comparison

Example: During a biceps curl, the biceps brachii acts as the agonist, the triceps brachii as the antagonist, and the brachialis as a synergist.